dispatch_source_cancel on a suspended timer causes EXC_BAD_INSTRUCTION

Question

I'm trying to cancel and then release a suspended timer but when I invoke 'dispatch_release' on it, I immediately get EXC_BAD_INSTRUCTION.

Is this not a valid set of actions to take on a timer?

Timer creation & suspension:

@interface SomeClass: NSObject { }
@property (nonatomic, assign) dispatch_source_t             timer;
@end

// Class implementation
@implementation SomeClass

@synthesize timer = _timer;

- (void)startTimer 
{
    dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 
                                    0, 0, globalQ); 

    dispatch_time_t startWhen = dispatch_walltime(DISPATCH_TIME_NOW, NSEC_PER_SEC * 1);
    dispatch_source_set_timer(_timer, startWhen, 1 * NSEC_PER_SEC, 5000ull);

    dispatch_source_set_event_handler(_timer, ^{
        // Perform a task 

        // If a particular amount of time has elapsed, kill this timer
        if (timeConstraintReached)
        {
            // Can I suspend this timer within it's own event handler block?
            dispatch_suspend(_timer);
        }
    });

    dispatch_resume(_timer);
}

- (void)resetTimer
{
    dispatch_suspend(_timer);

    dispatch_source_cancel(_timer);

    // dispatch_release causes 
    // 'EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    dispatch_release(_timer);

    self.timer = nil;    
}
@end

Additionally, can I invoke dispatch_suspend within a timer source's event_handler block?

Any help would be appreciated.

Answer 1

The reason it crashes is because of this code:

void
_dispatch_source_xref_release(dispatch_source_t ds)
{
    if (slowpath(DISPATCH_OBJECT_SUSPENDED(ds))) {
        // Arguments for and against this assert are within 6705399
        DISPATCH_CLIENT_CRASH("Release of a suspended object");
    }
    _dispatch_wakeup(ds);
    _dispatch_release(ds);
}

So, you can't release a dispatch_source_t that has been suspended. You probably want to just not suspend it in resetTimer I guess.

Whilst I can't find anything in the docs for why they have written it like this (and the comment alludes to the pros and cons being in a radar we'll never see), all I can do is refer to the docs where it says:

You can suspend and resume the delivery of dispatch source events temporarily using the dispatch_suspend and dispatch_resume methods. These methods increment and decrement the suspend count for your dispatch object. As a result, you must balance each call to dispatch_suspend with a matching call to dispatch_resume before event delivery resumes.

Whilst that doesn't say you can't release a dispatch source that's been suspended, it does say you have to balance each call so I'm assuming it's something along the lines of it's using a dispatch semaphore under-the-hood which have to be balanced before they can be released. That's just my guess though :-).

As for "can I invoke dispatch_suspend within a timer source's event_handler block". I'm pretty sure you can, yes, as per the docs for dispatch_suspend:

The suspension occurs after completion of any blocks running at the time of the call.